Start-up procedure for refrigerant systems having microchemical consensor and reheat cycle

ABSTRACT

A refrigerant system has a condenser of microchannel design and construction and includes a reheat cycle. The reheat cycle includes a refrigerant flow control device, such as a three-way valve, for selectively routing at least a portion of refrigerant through a reheat heat exchanger from a location between a compressor and expansion device. A control for the refrigerant system selectively actuates this refrigerant flow control device to route at least a portion of refrigerant through the reheat heat exchanger at system start-up.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/061,142, which was filed Jun. 13, 2008.

BACKGROUND OF THE INVENTION

Refrigerant systems utilize a refrigerant to condition a secondaryfluid, such as air, delivered to a climate-controlled space. In a basicrefrigerant system, the refrigerant is compressed in a compressor, andflows downstream to a condenser in a subcritical refrigerant cycle or toa gas cooler in a transcritical refrigerant cycle, where heat istypically rejected from the refrigerant to ambient environment, duringheat transfer interaction with this ambient environment. Thenrefrigerant flows through an expansion device, where it is expanded to alower pressure and temperature, and to an evaporator, where during heattransfer interaction with a secondary fluid (e.g., indoor air), therefrigerant is evaporated and typically superheated, while cooling andoften dehumidifying this secondary fluid.

In recent years, much interest and design effort has been focused on theefficient operation of the heat exchangers (condenser and evaporator) ofthe refrigerant systems. One relatively recent advancement in heatexchanger technology is the development and application of parallelflow, or so-called microchannel or minichannel, heat exchangers (thesetwo terms will be used interchangeably throughout the text), as thecondensers and evaporators.

These heat exchangers are provided with a plurality of parallel heatexchange tubes, typically of a non-round shape, among which refrigerantis distributed and flown in a parallel manner. The heat exchange tubesare orientated generally substantially perpendicular to a refrigerantflow direction in the inlet, intermediate and outlet manifolds that arein flow communication with the heat exchange tubes. The heat exchangetubes typically have a multi-channel construction, with refrigerantdistributed and flowing within these multiple channels in a parallelmanner. Heat transfer fins are inter-disposed in between and rigidlyattached to heat exchange tubes. The primary reasons for the employmentof the parallel flow heat exchangers, which usually have aluminumfurnace-brazed construction, are related to their superior performance,high degree of compactness, structural rigidity, lower weight, lowerrefrigerant charge and enhanced resistance to corrosion.

One concern with utilizing microchannel heat exchangers, also related totheir advantage, is their low internal volume. Due to low internalvolume, microchannel heat exchangers are more susceptible to refrigerantpressure variations due to instantaneous changes in refrigerant flowthroughout the refrigerant circuit. Microchannel heat exchangers arealso very sensitive to refrigerant charge amounts, with even a smallamount of extra refrigerant charge in the system leading to higher thandesirable discharge operating pressures and instantaneous pressurespikes. These problems are especially pronounced during start-ups.Nuisance interruptions of the refrigerant system operation can be aresult of emergency shutdown by control software on a high pressurealarm or by mechanical safety, such as a high pressure switch, leadingto complete inability to operate the refrigerant system, if a dischargepressure spike exceeded predetermined allowable safe limit (typicallyfor a preset number of times). This consequently would results in afailure to keep a climate-controlled environment within desirabletemperature and humidity ranges, leading to occupant discomfort andliability claims. Under certain circumstances, repeated starts andshutdowns in short periods of time can potentially lead to a compressorfailure.

Another refrigerant cycle component is a reheat cycle utilizing primaryrefrigerant circulating throughout the main refrigerant circuit. In thereheat cycle, at least a portion of refrigerant passes through a reheatheat exchanger which is positioned in the path of air flowing across theevaporator. The reheat heat exchanger is typically positioned in thepath of the air downstream of the evaporator. With a reheat cycleactuated, air can be cooled in the evaporator below normally desirabletemperature, allowing for a greater amount of moisture removal from theair stream. The air then passes over the reheat heat exchanger and isheated back toward the target temperature. Typically, reheat cycles areprovided with a refrigerant flow control device, such a three-way valve,that can selectively route at least a portion of refrigerant through thereheat heat exchanger when reheat is desired. The reheat cycle has notbeen operated at system start-up, in order to prevent high pressurespikes and nuisance refrigerant system shutdowns.

SUMMARY OF THE INVENTION

A refrigerant system has a compressor delivering a compressedrefrigerant to a condenser. Refrigerant from the condenser passesthrough an expansion device and an evaporator. From the evaporator it isreturned to the compressor. The condenser is a microchannel heatexchanger. A reheat cycle includes a refrigerant flow control device forselectively routing at least a portion of refrigerant through a reheatheat exchanger. The reheat heat exchanger is positioned in a path of airthat has passed over the evaporator. A control for the refrigerantsystem selectively actuates a switch to route refrigerant through areheat heat exchanger at system start-up.

In one embodiment, the reheat cycle may also be actuated at certainenvironmental and operating conditions, when high pressure spikes areexpected to occur. Such conditions may include, for instance, highambient temperatures, higher operating speeds of variable speedcompressors and a higher number of active tandem compressors. Theseenvironmental and operating conditions may be pre-programmed and storedin the memory of the refrigerant system controller.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a first embodiment schematic.

FIG. 1B shows an alternative embodiment.

FIG. 2A shows an exemplary microchannel heat exchanger.

FIG. 2B is a cross-section through a portion of FIG. 2A.

FIG. 3 is a graph showing start-up utilizing the disclosed method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant system 20 is illustrated in FIG. 1A and includes acompressor 22 delivering refrigerant into a discharge line heading to acondenser 24. The condenser 24 is a parallel flow heat exchanger, and inone disclosed embodiment is a microchannel or minichannel heatexchanger. As mentioned above, these terms are used interchangeablyhere.

Heat is transferred in the condenser 24 from the refrigerant to asecondary fluid, such as ambient air. The high pressure, desuperheated,condensed and typically subcooled, refrigerant passes from the condenser24 into an expansion device 38, where it is expanded to a lower pressureand temperature. Downstream of the expansion device 38, refrigerantflows through an evaporator 36 and back to the compressor 22. As known,the heat exchanger 24 operates as a condenser in subcriticalapplications and as a gas cooler in transcritical applications.Nevertheless, although both applications are within the scope of theinvention, the heat exchanger 24 will be referred throughout the text asa condenser.

A reheat cycle is incorporated into the refrigerant system 20. As known,a refrigerant flow control device such as a three-way valve 30selectively routes at least a portion of refrigerant downstream of thecondenser 24 and through a reheat heat exchanger 32. An air-movingdevice such as a fan 34 blows air over an evaporator 36, and over thereheat heat exchanger 32. That is, the reheat heat exchanger 32 ispositioned indoors, along with the evaporator 36, and downstream, withrespect to the air flow, of the evaporator 36. As mentioned above,essentially, the reheat cycle is selectively actuated by opening (fullyor partially) the three-way valve 30 to direct at least a portion ofrefrigerant through the reheat heat exchanger 32 when dehumidificationin a climate-controlled environment X is desired. Under suchcircumstances, the refrigerant system is controlled such that theevaporator 36 cools the air to a temperature below that is desired inthe environment to be conditioned X, which allows removing an additionalamount of moisture from the air to be delivered to the conditionedenvironment X. As the air passes over the reheat heat exchanger 32, itis reheated toward the target temperature. As a result, temperature andhumidity control are achieved in the climate-controlled environment X.

Downstream of the reheat heat exchanger 32, there is an optional checkvalve 40. Further, as shown, a condenser bypass line 26 selectivelybypasses at least a portion of refrigerant around the condenser 24 andincludes a refrigerant flow control device such as a valve 28. Thisallows for achieving variable dehumidification capability, or variablesensible heat ratios. The valve 28 can be adjustable (through modulationor pulsation) or of an on/off type.

FIG. 1B shows an alternative embodiment wherein the reheat cyclethree-way valve 42 is positioned upstream of the condenser 24 anddelivers at least a portion of refrigerant through a reheat refrigerantline 44 to a reheat heat exchanger (not shown). For purposes of thisapplication, the exact location of the three-way valve 42 and the reheatheat exchanger 32 is not critical, provided they are both located on thehigh pressure side of the refrigerant system 20. Also, as known, thethree-way valves 30 and 42 can be replaced by a pair of conventionalvalves performing identical refrigerant routing function.

As shown in FIG. 2A, an inlet line 146 downstream of the compressor 22delivers refrigerant into a first bank of parallel heat exchange tubes148, and then across the condenser core to a first chamber of anintermediate manifold structure 133. From the intermediate manifoldstructure 133, the refrigerant passes back through a second bank ofparallel heat exchange tubes 150 to an intermediate chamber in themanifold 147. Refrigerant then passes through yet another bank ofparallel heat exchange tubes 152, returning to the intermediate manifold133. From the intermediate manifold 133, the refrigerant passes throughanother bank of heat exchange tubes 154 back to the manifold 147, and anoutlet refrigerant line. Of course, this is simply one illustratedembodiment. It should be noted that, in practice, there may be more orless refrigerant passes than the four illustrated passes 148, 150, 152,and 154. Further, it should be understood that, although for simplicitypurposes, each refrigerant pass is represented by a single heat exchangetube, typically there are many heat exchange tubes within each passamongst which refrigerant is distributed while flowing within the pass.In condenser applications, a number of the heat exchange tubes withineach bank may decrease in a downstream direction, with respect torefrigerant flow. For instance, there could be 12 heat exchange tubes inthe first bank, 8 heat exchange tubes in the second bank, 5 heatexchange tubes in a third bank and only 2 heat exchange tubes in thelast forth bank. Separator plates 143 are placed within the manifolds133 and 147 to separate the chambers positioned within the same manifoldstructure.

As shown in FIG. 2B, the heat exchange tubes within the tube banks 148,150, 152, and 154 may consist of a plurality of parallel channels 100separated by walls 101. The FIG. 2B is a cross-sectional view of theheat exchange tubes shown in FIG. 2A. The channels 100 allow forenhanced heat transfer characteristics and assist in improved structuralrigidity of the heat exchanger. The cross-section of the channels 100may take different forms, and although illustrated as a rectangular inFIG. 2B, may be, for instance, of triangular, trapezoidal, oval orcircular configurations. The size of the channels 100 in a microchannelheat exchanger is quite small. As disclosed, the channels could have ahydraulic diameter of less than or equal to 5 mm, and more narrowly,less than or equal to 3 mm. Notably, the use of “hydraulic diameter”does not imply the channels are circular.

As mentioned above, when microchannel heat exchangers are utilized ascondensers, pressure spikes which can be particularly observed at therefrigerant system start-up, can provide a challenge to a refrigerantsystem designer. One concern with microchannel heat exchangers is thattheir internal volume is relatively small, and thus they areparticularly susceptible to pressure spikes and extremely sensitive torefrigerant charge amounts. Although pressure spikes are particularlypronounced at refrigerant system start-up, they can be also observed atchanges of operating conditions such as, for instance, a sharp increaseof the compressor speed or activating a larger number of tandemcompressors, in order to satisfy thermal load demands in the conditionedspace X.

In this invention, the reheat circuit is actuated at refrigerant systemstart-up. Now, when the refrigerant is passing through both thecondenser 24, and through the reheat heat exchanger 32, there is alarger combined internal volume on a high pressure side of therefrigerant system, and the amplitude of the pressure spike is thusreduced. In some instances, all of the refrigerant could pass throughthe reheat heat exchanger 32.

As shown in FIG. 3, with a conventional start-up S, and without thereheat circuit being actuated, a pressure spike can be relatively high,and may exceed the safety limit Y. With the present application, and asshown at Z in FIG. 3, the amplitude of the pressure spike is greatlyreduced, due to the combined internal volume of the heat exchangers 24and 32. In this manner, the pressure spike may well be below the safetylimit Y, and nuisance shutdowns, caused by control software operating ona high pressure alarm or by mechanical safety, such as a high pressureswitch, can be avoided. This provides uninterrupted control oftemperature and humidity within the desired ranges and occupant comfortin the climate-controlled environment. Furthermore, repeated starts andshutdowns of the refrigerant system in short periods of time will beavoided, leading to improved compressor reliability andtemperature/humidity variation reduction in the conditioned space.

The reheat heat exchanger 32 can be any type of a heat exchanger,including standard heat exchangers or a microchannel heat exchanger.

A control 110 for the refrigerant system may be of any appropriateelectronic control type, as is known in the art. The control wouldtypically control all system components, and not only the three-wayvalve 30 that can be adjustable (through modulation or pulsation) or ofan on/off type. The control 110 can actuate the reheat cycle at certainenvironmental and operating conditions, when high pressure spikes arelikely to occur. Such conditions may include, for instance, high ambienttemperatures, higher operating speeds of variable speed compressors anda higher number of active tandem compressors. These environmental andoperating conditions may be pre-programmed and stored in the memory ofthe refrigerant system control 110. Further, under some environment andoperating conditions, it may be that there is less likelihood of apressure spike at system start-up. Thus, the control may be programmedto not actuate the reheat cycle in these instances.

In addition, after some period of time following refrigerant systemstart-up, the three-way valve 30 is deactivated to block flow ofrefrigerant through the reheat heat exchanger 32, unlessdehumidification mode of operation is desired. This period of time canbe on the order of 15 seconds to 3 minutes.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A refrigerant system comprising: a compressor for delivering acompressed refrigerant to a condenser, refrigerant from said condenserpassing through an expansion device, and from said expansion devicethrough an evaporator, and from said evaporator being returned to saidcompressor; and said condenser being a microchannel heat exchanger; areheat cycle including a refrigerant flow control device for selectivelyrouting at least a portion of refrigerant through a reheat heatexchanger, said refrigerant flow control device being positioned toroute said at least portion of refrigerant through said reheat heatexchanger from a location between said compressor and said expansiondevice, and said reheat heat exchanger being positioned in a path of airthat has passed over said evaporator; and a control for the systemselectively actuating said switch to route at least a portion ofrefrigerant through said reheat heat exchanger at refrigerant systemstart-up.
 2. The refrigerant system as set forth in claim 1, whereinsaid control is also routing said at least a portion of refrigerantthrough said reheat heat exchanger when at least one of a start-up,compressor speed change, tandem compressor activation or high ambienttemperature conditions occur.
 3. The refrigerant system as set forth inclaim 2, wherein said conditions are programmed in the control toidentify when to selectively actuate said refrigerant flow controldevice to route refrigerant through said reheat heat exchanger.
 4. Therefrigerant system as set forth in claim 1, wherein a bypass is providedaround said condenser to selectively bypass at least a portion ofrefrigerant around said condenser.
 5. The refrigerant system as setforth in claim 1, wherein said refrigerant flow control device routessaid at least portion of refrigerant from a location between saidcompressor and said expansion device, and downstream of said compressor.6. The refrigerant system as set forth in claim 1, wherein saidrefrigerant flow control device is actuated to selectively allow forrefrigerant flow through the reheat heat exchanger for at least apredetermined period of time after said condition is identified.
 7. Therefrigerant system as set forth in claim 6, wherein said predeterminedperiod of time is preferably from 15 seconds to 3 minutes.
 8. Therefrigerant system as set forth in claim 1, wherein said refrigerantflow control device is one of adjustable type through modulation orpulsation or of an on/off type.
 9. The refrigerant system as set forthin claim 1, wherein said microchannel heat exchanger includes aplurality of heat exchange tube each having a plurality of parallelrefrigerant channels.
 10. The refrigerant system as set forth in claim9, wherein said microchannel heat exchanger having flow channels with ahydraulic diameter less than or equal to 5 mm.
 11. The refrigerantsystem as set forth in claim 1, wherein all of the refrigerant passesthrough said reheat heat exchanger.
 12. A method of operating arefrigerant system comprising the steps of: a) delivering a compressedrefrigerant to a condenser, refrigerant from said condenser passingthrough an expansion device, and from said expansion device through anevaporator, and from said evaporator being returned to said compressor;b) said condenser being a microchannel heat exchanger; c) selectivelyrouting at least a portion of refrigerant through a reheat heatexchanger from a location between said compressor and said expansiondevice, and passing at least a portion of air over said reheat heatexchanger after the air has passed over said evaporator; and d)selectively actuating a refrigerant flow control device to route said atleast portion of refrigerant through said reheat heat exchanger atsystem start-up.
 13. The method as set forth in claim 12, furthercomprising the step of routing said at least a portion of refrigerantthrough said reheat heat exchanger when at least one of a start-up,compressor speed change, tandem compressor activation or high ambienttemperature occurs.
 14. The method as set forth in claim 12, furthercomprising the step of selectively allowing refrigerant flow through thereheat heat exchanger for at least a predetermined period of time afterrefrigerant system start-up.
 15. The method as set forth in claim 12,wherein all of the refrigerant passes through said reheat heatexchanger.